1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine, incorporating such a fixing device.
2. Description of the Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
One such fixing device includes a multi-roller, belt-based fuser assembly that employs an endless, flexible fuser belt entrained around multiple rollers, one of which is equipped with an internal heater to heat the length of the fuser belt through contact with the heated roller. The fuser belt is paired with a pressure roller pressed against the outer surface of the fuser belt to form a fixing nip therebetween, at which a toner image is fixed in place with heat from the fuser belt and pressure from the pressure roller.
Owing to the fuser belt which exhibits a relatively low heat capacity and therefore can be swiftly heated, the belt-based fuser assembly eliminates the need for keeping the heater in a sufficiently heated state when idle, resulting in shorter start-up time and smaller amounts of energy wasted during standby, as well as a relatively compact size of the fuser assembly.
One important factor that determines imaging quality of a fixing device is the ability to properly convey a recording medium through the fixing nip without causing the recording medium to wrap around the rotary fixing member. Media wraparound occurs where the toner image heated through the fixing nip becomes sticky and thus adheres to the surface of the fixing member upon exiting the fixing nip. If not corrected, a recording medium wrapping around the fixing member would cause jam or other conveyance failure in the fixing nip.
For obtaining a fixing process with high immunity against media wraparound and concomitant conveyance failure, a fixing device may use a fuser roller or belt coated with a release agent such as fluorine resin where it contacts a heated, sticky toner image in the fixing nip, while equipped with a media stripping mechanism that allows a recording medium to properly separate from the fuser member at the exit of the fixing nip.
For example, in monochrome printing, a fuser roller includes a cylindrical body of metal coated with polytetrafluoroethylene (PTFE) commercially available under the trademark Teflon®. The metallic fuser roller is durable and highly immune to abrasion or other damage caused by continuous contact with the media stripping mechanism.
On the other hand, in multi-color printing, a fuser roller often includes a cylindrical body covered by an outer elastic layer of silicone rubber or the like with a coating of oil or fluorine resin deposited thereon (e.g., a rubber-covered roller with a several tens micron-thick tubular coating of perfluoroalkoxy fitted thereupon). The rubber-covered roller allows for good reproduction of color, while relatively vulnerable to damage as the outer elastic layer readily abrades due to continuous contact with the media stripping mechanism, resulting in streaks or other imperfections in a resulting image.
A practical approach to prevent damage to the fuser member and concomitant image defects is to use a non-contact media stripping mechanism that can strip a recording medium without touching the fuser member. One example of such non-contact media stripper is an elongated mechanical assembly, such as a thin-edged stripping plate or a plurality of interspersed stripping fingers arranged in line, which extends parallel to a length of the fuser member with a spacing of approximately 0.2 mm to approximately 1.0 mm left between the separator and the fuser member. Another example is a self-stripping system that optimizes curvature and elasticity of a fixing member forming a fixing nip to promote self-stripping action, in which a recording medium having a specific stiffness or beam strength separates by itself from the fixing member.
Although effective for their intended purposes, the non-contact stripping methods depicted above would not work properly, where the recording medium in use exhibits a greater than usual tendency to follow or adhere to the fixing member, and eventually enters a gap, if any, left between the fixing member and an adjoining guide or stripper structure. Such deflection of a recording medium from a proper conveyance path would cause the recording medium to wrap around the fuser member, or otherwise unduly interfere with the media stripper, resulting in jam and other concomitant conveyance failure in the fixing nip.
To date, a still another non-contact stripping method is proposed, which employs a pneumatic nozzle that directs air to an interface between a fuser member and a printed face of a recording medium, so as to pneumatically force the recording medium to separate from the fuser member at the exit of a fixing nip. Such pneumatic media stripper is designed with a sufficiently large pneumatic stripping force (which may be determined, for example, by pressure and amount of air discharged from the nozzle) to accommodate a most difficult situation where the recording medium has a pronounced tendency to follow or adhere to the fixing member, so as to ensure reliable stripping performance for normal, easier conditions under which the fixing device is normally operated.